[v2,11/16] fanotify: disable readahead if we have pre-content watches

Commit Message

Josef Bacik Aug. 8, 2024, 7:27 p.m. UTC

With page faults we can trigger readahead on the file, and then
subsequent faults can find these pages and insert them into the file
without emitting an fanotify event.  To avoid this case, disable
readahead if we have pre-content watches on the file.  This way we are
guaranteed to get an event for every range we attempt to access on a
pre-content watched file.

Signed-off-by: Josef Bacik <[e-mail]>
---
 mm/filemap.c   | 12 ++++++++++++
 mm/readahead.c | 13 +++++++++++++
 2 files changed, 25 insertions(+)

Comments

Christian Brauner Aug. 9, 2024, 12:12 p.m. UTC | #1

On Thu, Aug 08, 2024 at 03:27:13PM GMT, Josef Bacik wrote:
> With page faults we can trigger readahead on the file, and then
> subsequent faults can find these pages and insert them into the file
> without emitting an fanotify event.  To avoid this case, disable
> readahead if we have pre-content watches on the file.  This way we are
> guaranteed to get an event for every range we attempt to access on a
> pre-content watched file.
> 
> Signed-off-by: Josef Bacik <[e-mail]>
> ---

Looks sensible,
Reviewed-by: Christian Brauner <[e-mail]>

Patch

diff --git a/mm/filemap.c b/mm/filemap.c
index ca8c8d889eef..8b1684b62177 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3122,6 +3122,14 @@  static struct file *do_sync_mmap_readahead(struct vm_fault *vmf)
 	unsigned long vm_flags = vmf->vma->vm_flags;
 	unsigned int mmap_miss;
 
+	/*
+	 * If we have pre-content watches we need to disable readahead to make
+	 * sure that we don't populate our mapping with 0 filled pages that we
+	 * never emitted an event for.
+	 */
+	if (fsnotify_file_has_pre_content_watches(file))
+		return fpin;
+
 #ifdef CONFIG_TRANSPARENT_HUGEPAGE
 	/* Use the readahead code, even if readahead is disabled */
 	if ((vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) {
@@ -3190,6 +3198,10 @@  static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
 	struct file *fpin = NULL;
 	unsigned int mmap_miss;
 
+	/* See comment in do_sync_mmap_readahead. */
+	if (fsnotify_file_has_pre_content_watches(file))
+		return fpin;
+
 	/* If we don't want any read-ahead, don't bother */
 	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
 		return fpin;
diff --git a/mm/readahead.c b/mm/readahead.c
index 817b2a352d78..bc068d9218e3 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -128,6 +128,7 @@ 
 #include <linux/blk-cgroup.h>
 #include <linux/fadvise.h>
 #include <linux/sched/mm.h>
+#include <linux/fsnotify.h>
 
 #include "internal.h"
 
@@ -674,6 +675,14 @@  void page_cache_sync_ra(struct readahead_control *ractl,
 {
 	bool do_forced_ra = ractl->file && (ractl->file->f_mode & FMODE_RANDOM);
 
+	/*
+	 * If we have pre-content watches we need to disable readahead to make
+	 * sure that we don't find 0 filled pages in cache that we never emitted
+	 * events for.
+	 */
+	if (ractl->file && fsnotify_file_has_pre_content_watches(ractl->file))
+		return;
+
 	/*
 	 * Even if readahead is disabled, issue this request as readahead
 	 * as we'll need it to satisfy the requested range. The forced
@@ -704,6 +713,10 @@  void page_cache_async_ra(struct readahead_control *ractl,
 	if (!ractl->ra->ra_pages)
 		return;
 
+	/* See the comment in page_cache_sync_ra. */
+	if (ractl->file && fsnotify_file_has_pre_content_watches(ractl->file))
+		return;
+
 	/*
 	 * Same bit is used for PG_readahead and PG_reclaim.
 	 */